In an asynchronous wireless digital communications system, a preamble can be placed at the beginning of a transmission to help a receiver detect the presence of the transmission and to train receive circuitry to help optimize the reception of the transmission. The preamble may contain a specified sequence of information or values of certain length. The preamble may contain actual usable information that can provide information about the transmission. Alternatively, the preamble may contain no information regarding the transmission other than to inform the receiver of the presence of the transmission.
At a receiver, the receiver must be able to detect the presence of the preamble on the transmission medium (air for example) in order to train its receive circuitry and prepare to receive the remainder of the transmission. Since the communications system is asynchronous, the receiver does not know when the preamble will appear on the transmission medium. Therefore, the receiver may have to regularly check the transmission medium for the presence of the preamble. Once the receiver detects the presence of the preamble, the receiver can use the preamble to train its receive hardware and then prepare to receive the actual information carried in the transmission.
For example, in IEEE 802.11b compliant digital wireless communications systems, there can be two preambles differing from one another in length. A short preamble can start with a 56-bit sequence of values while a long preamble can start with a 128-bit sequence of values. Each bit can be scrambled by a scrambler which places a measure of randomness to the transmission. Note that for actual transmission purposes, these scrambled bits can be modulated (spread) with a spreading code. Therefore, when transmitted, each bit in the preamble can be represented by more than one value. According to the IEEE 802.11 technical standards, each bit in the preamble can be modulated by a pseudo-random number sequence commonly referred to as a Barker sequence. For example, an 11-value Barker sequence (B0=+1−1+1+1−1+1+1+1−1−1−1 or B1=−1+1−1−1+1−1−1−1+1+1+1) can be used to modulate the bits of the preamble, depending upon the value of the bit. In IEEE 802.11g compliant digital wireless communications systems there can be three preambles, one at each length as described above and a third for a different modulation type (orthogonal frequency division multiplexing (OFDM)).
However, detecting pseudo-random data sequences can be difficult, especially when the sequence being detected may be a random pattern of pseudo-random sequences (such as the Barker sequences) and may require the use of a significant amount of digital signal processing. For example, in an IEEE 802.11b compliant communications system, the scrambling effectively forces a receiver to detect an unknown pattern of Barker sequences (B0's and B1's). This signal processing can increase power consumption in the receiver, which can be a problem especially if the receiver is a battery powered device. A commonly used solution that has been used in the past to help in the detection of transmissions or to denote a special operating mode is the addition of a data field in the transmission. To assist in the detection of a transmission, the data field may contain an easy to detect sequence, while to denote a special operating mode, the data field may contain a specific value.
One disadvantage of the prior art is that the use of a pseudo-random initial state in a scrambler to scramble the bits in a preamble is that to a receiver, the transmitted preamble can appear to be noise on the transmission medium. Therefore, to be able to detect the preamble, the receiver may need to perform a significant amount of digital signal processing in order to differentiate between noise and a preamble. The need to perform digital signal processing in order to detect the preamble can imply that the receiver can not place its digital circuitry to sleep in order to reduce power consumption.
A second disadvantage of the prior art is that the use of an additional data field in a transmission can result in the inability of receivers which are adherent to a specific technical standard to understand the transmission. This may be a problem since it can be difficult to market a communications system that is incompatible (and hence, unusable) with a widely accepted technical standard.
A third disadvantage of the prior art is that the use of an additional data field may not be able to assist in simplifying the detection of the transmission if the additional data field is itself scrambled. If the additional data field is scrambled, then it could also appear as noise to a receiver.